The present invention relates, in general terms, to torsion-damping devices of the type comprising at least two coaxial parts, one exciting and the other excited, which are mounted to rotate relative to one another within the limits of a specific angular movement, and elastic means interposed circumferentially between said parts.
As is known, such a torsion-damping device is conventionally used in the construction of a clutch plate assembly, especially for a motor vehicle, in which case one of its rotary parts carries a friction disc intended to be made to rotate integrally with a first shaft, in practice a driving shaft, the output shaft of the engine in the case of a motor vehicle, and thus forms an exciting part, whilst the other of its rotary parts is carried by a hub intended to be made to rotate integrally with a second shaft, in practice a driven shaft, the input shaft of a gearbox in the case of such a motor vehicle, and thus forms an excited part.
Such a device makes it possible, in fact, to ensure regulated transmission of the torque applied to one of its rotary parts when the other is itself subjected to a torque, that is to say to filter the vibrations capable of arising over the entire length of the kinematic chain in which it is inserted and which extends from the engine to the controlled wheel shafts in the case of a motor vehicle.
Most often, the elastic means interposed circumferentially between the two rotary parts of such a torsion-damping device comprise conventionally a plurality of springs which all extend substantially tangentially to a circumference of the assembly and which are each located partly in a housing formed in a first of these rotary parts and partly in a housing formed in the second of these rotary parts, arrangements being made to ensure that these springs intervene in stages during successive portions of the angular movement between said rotary parts, for the purpose of an increase, modulated in a specific way, in the overall rigidity due to these springs, this rigidity having to be relatively low for low torques at the start of the angular movement and relatively high for high torques at the end of this angular movement.
In practice, these arrangements result from the fact that at least one of the springs in question is, as regards the configuration of the rest of the assembly, located without any circumferential play in the corresponding housing of a first of the rotary parts, and with a circumferential play J1 for a first direction of rotation of said assembly and J2 for the opposite direction of rotation, in the corresponding housing of the second of said rotary parts.
In fact, in a first stage, and therefore for low torques, this spring has no effect, and it comes into action only in a second stage after the corresponding circumferential play J1 or J2 has been absorbed.
Thus, in a first stage, the only springs which may be capable of intervening are those which, in practice, are springs of relatively low rigidity, whilst, in a second stage, the effects of springs of relatively high rigidity are added to the effects of these springs of relatively low rigidity.
At the same time, friction means interposed between the two rotary parts in question intervene.
These friction means conventionally comprise at least one friction washer which is driven to rotate with one of said rotary parts through at least one range of the angular movement between these, and which is in contact with the other of said rotary parts with at least one of its faces.
The purpose of these friction means is to introduce a "hysteresis" effect into the development of the torque transmitted from one of the rotary parts to the other as a function of the angular movement between them, that is to say, to lead, for a given value of this movement, to a differentiation between the value of the corresponding torque for an increasing development of this torque and the value of this torque for a decreasing development thereof, this being favorable to the absorption of energy necessary for the desired filtration.
A dual problem has to be overcome in the production of torsion-damping devices of this type.
First of all, it is desirable that the hysteresis introduced by the friction means used should differ according to the rigidity of the active elastic means.
In practice, this hysteresis can be substantial only when the active elastic means have high rigidity, since the energy to be absorbed is then substantial itself.
On the other hand, when the active elastic means have only relatively low rigidity, this being the case, as indicated above, for low torque values, it is preferable for the hysteresis generated by the friction means used to be low, otherwise there is a risk of "cancelling" or "suppressing" these elastic means, preventing them from acting.
Furthermore, and in conjunction with this, the range of angular movement during which only elastic means of low rigidity are therefore active, as required for good filtration of noises, called neutral noises, which are generated during idling of the engine of a stationary vehicle, with the gearbox in neutral, generally cannot be as wide as desired both because the extension of such a range comes up against the requirements of mechanical construction and bulk of the torsion-damping device in question, within which the available circular space necessary for such an extension is inevitably limited, and because, outside the idling operation for which they are thus provided, these elastic means of low rigidity cause, at each momentary release of the accelerator pedal and/or at each change of gear, an unpleasant "rocking" effect.
Now, in practice, at idling speed, and assuming the amplitude of the oscillations of the exciting rotary part to be constant with the temperature, the corresponding oscillations of the excited part are generated on either side of a mean position which does not correspond to a zero angular movement between said parts, because a parasitic torque, called a drag torque, exists, on the contrary, between these parts and leads to the presence of a non-zero angular movement between them.
This drag torque which is relatively large in cold running decreases with temperature.
In practice, only a portion of the range of angular movement corresponding to the elastic means of low rigidity is actually used for the oscillations of the exciting part at idling speed.
However, the result of the drag torque which exists at that time is that the location of this portion in this range varies with the temperature.
If this drag torque is sufficiently large, this being the case in cold running, and if, for the reasons set out above, the extension of the range of angular movement corresponding to the elastic means of low rigidity is limited, it can happen that, at idling speed, the oscillations of the excited part lead the latter to come in contact with the elastic means of high rigidity, which are not normally intended to come into action until the next range of this angular movement.
The abrupt increase in rigidity which then occurs never fails to result in an accelerated backward rebound of the excited part which leads the latter to move prematurely not only over a portion of the range of the angular movement corresponding to the elastic means of low rigidity, according to oscillations caused simply by those of the exciting part, but over the whole of this range, in successive rebounds to the ends of this, a pronounced specific noise being produced.
The subject of the present invention is, in general terms, an arrangement which makes it possible, in a very simple way, to mitigate the consequences of such a rebound, and which also leads to other advantages.